WoW Census

The WoW census showed the total number of players in each faction, per each race, and per each class who self-reported via the CensusPlusAddOn, which scans players around the mod's player character. The Census only includes characters that are level 10 or higher and that have been active within the past 30 days.

Unfortunately, the popularity of this AddOn has gone down dramatically since 2010, so it's data is mostly only useful for ratios.

Quite interestingly, according to these records, Human Paladins are the most played race/class combination for the Alliance and Blood Elf Warlocks are most played race/class combination for the Horde. Though the Shaman and Warlock percentages are equal, Blood Elves (the most played Horde race) cannot be Shaman and thus Blood Elf Warlocks are technically the most common.